Cooperative wireless communication systems are known to provide larger coverage and lesser sensitivity to fading than conventional cellular systems. A description of wireless systems of the cooperative type is for instance to be found in the article by J. N. Laneman and G. W. Wornell, entitled “Exploiting distributed spatial diversity in wireless networks”, published October 2000 in Proc. Allerton Conf. on Comm., Contr. and Computing.
FIG. 1A schematically illustrates a first type of cooperation policy in such a system. The source terminal s transmits a data flow to a relay terminal r. The relay terminal receives the data flow and forwards it, possibly after decoding and recoding the same, to the destination terminal s. This policy enables the source terminal, e.g. a base station of a cellular network, to reach the terminal d when the path s-d is not line of sight (NLOS), because of the presence of an obstacle between s and d, or if the distance between s and d exceeds the range of the base station. The channel s-r-d resulting from concatenation of channels s-r and r-d is called relayed channel.
FIG. 1B schematically illustrates a second cooperation policy in such a system. The source terminal s transmits a data flow to a destination terminal d. The relay terminal r also receives the data flow from s and relays it to the destination terminal d. Thus, the destination terminal d receives a data flow via the direct channel s-d and via the relayed channel s-r-d. It may use the direct flow from s or the relayed flow from r, or even combine both of them to take advantage of diversity combining, as will be explained below.
Furthermore, the two cooperation policies set forth above can make use of a single relay or a plurality of relays. In the most general case, the cooperative network can be considered a graph having a source node (which is the source terminal), a destination node or sink (which is the destination terminal), and a plurality of intermediate nodes acting as relay terminals.
When a data flow is relayed in one or several times between a source terminal and a destination terminal, in other words, if there is a directed path in the graph between the source node and the destination node, going through at least one intermediate node, each component segment of this path is called a hop. Thus, in the simple case of FIG. 1A, the link between s and d is a so-called double hop link, and more generally speaking a multiple hop link. In the case of FIG. 2B, the presence both of a single hop link s-d and of a multiple hop link s-r-d is to be noted.
A multiple hop link makes use of a plurality of successive transmission channels. The resulting channel is then called a multiple hop channel. Thus, the relayed transmission channel s-r-d consisting of channels s-r and r-d is a 2 hop channel.
In any case, after amplification, a relay terminal may simply forward the signal it receives from the source terminal (so-called AF protocol, for Amplify and Forward) or else previously decode the signal received before forwarding (so-called DF protocol, for Decode and Forward) in a recoded form. When the code used for recoding is the same as the one used by the decoding, it is a DF protocol with constrained coding (or repetition code). Of course, in the opposite case, coding is said to be non-constrained.
In addition, OFDMA-type communication systems are well known in the state of the art, e.g. WiMax (802.16) or 3GP/LTE (3GPP Long Term Evolution) systems. In such a system, various users are assigned distinct subcarrier intervals (or frequency chunks) of an OFDM multiplex. The allocation of subcarrier intervals to a user is in general done dynamically for each transmission interval. Thus, from one transmission interval to the other, a base station transmitting data flows to several terminals may use a different allocation of subcarrier intervals for transmitting such flows. In a transmission interval, the emitter (e.g. a base station) transmits a sequence of OFDM symbols carrying the flow to the different users.
Use of a cooperative policy in an OFDMA network is also known in the state of the art. Thus, the standardization project IEEE 802.16j proposes to apply a cooperative policy to a cellular network of the WiMax type.
Different cooperative transmission protocols may be envisaged. For the sake of simplicity, the presentation thereof is limited to the simple case of a network comprising a source terminal, a relay terminal, and a destination terminal. All of these protocols share the implementation of two consecutive transmission intervals, a first interval during which the source terminal is transmitting and the relay terminal is receiving, and a second interval during which the relay terminal is forwarding and the destination terminal is receiving. However, they are different in that direct transmission between the source terminal and the destination terminal can take place either during the first transmission interval, or during the second one, or even during both of them.
FIGS. 2A to 2C illustrate three transmission protocols. Respectively, simple lines and double lines illustrate transmissions taking place during the first and second transmission intervals.
According to a first protocol, illustrated in FIG. 2A, the source terminal is transmitting to the relay and destination terminals during a first transmission interval, and the relay terminal is forwarding to the destination terminal during a second transmission interval.
According to a second protocol, illustrated in FIG. 2B, the source terminal is transmitting to the relay terminal during the first transmission interval. The source terminal is transmitting and the relay terminal is forwarding to the destination terminal during the second transmission interval.
Finally, according to a third protocol illustrated in FIG. 2C, the source terminal is transmitting to the relay and destination terminals during the first transmission interval. The source terminal is transmitting and the relay terminal is forwarding to the destination terminal during the second transmission interval.
The first protocol is said to be an orthogonal cooperation protocol, in as far as the signals received by the recipient are separated by distinct transmission intervals. More in general, if the destination terminal receives signals from a source and from one or several relays, by means of orthogonal resources (frequencies, subcarrier intervals, codes, transmission intervals), cooperation will still be orthogonal. The destination terminal may or may not combine the signals received and relayed on the orthogonal resources.
Herein, it is understood that a combination is either a simple average of the soft values of the data received on the two orthogonal resources (Diversity Combining) in case of a DF protocol with repetition code, or a code combining method, in case of a non-constrained DF protocol. Code combination may be a simple code concatenation (e.g. a code word transmitted on different resources), or use turbo decoding (the source and relay terminals acting as the elementary encoders of a space distributed turbo encoder).
Conversely, for the transmission protocols of FIGS. 2B and 2C, cooperation is said to be non-orthogonal if the same transmission resources, here the same transmission interval and, in an OFDMA system, the same subcarrier interval, are used by the relay and source terminals. Thus, the destination terminal receives, on the same transmission resource, the direct signal from the source terminal and the signal relayed by the relay terminal, in a combined form.
In addition, the forwarding request protocols ARQ (Automatic Repeat reQuest) and HARQ (Hybrid ARQ) are known in point-to-point telecommunication systems. In such a system, bit packets are encoded by an emitter by means of so-called FEC (Forward Error Correction) coding, which may adopt the form of an error detection code (CRC) or an error correction code (ECC), before being modulated and transmitted. If the receiver detects in a packet the presence of an error which it cannot correct, it will transmit a negative acknowledgement (NACK) to the emitter. The combination of FEC coding and of an ARQ forwarding protocol is known in literature by the acronym HARQ (Hybrid ARQ). Presently, there are several versions of the HARQ protocol.
In the simplest version, the so-called HARQ Type I, when an error cannot be corrected in a packet, a request for forwarding the incorrect packet is sent to the emitter and a second attempt of transmission occurs. In practice, the incorrect packet is not deleted, but is stored in a receiving buffer so as to be combined (Chase combining) with the block received at the second attempt. The result of the combination is then submitted to the decoder.
A second version of the HARQ protocol, the so-called HARQ Type II or IR HARQ (Incremental Redundancy HARQ) allows for the size of the forwarded packets to be reduced. According to this version, in the first transmission of the packet, the code is punctured so that few redundancy bits are transmitted. If the packet received in the first transmission turns out to be incorrect and cannot be corrected by means of redundancy bits available, additional redundancy bits, eliminated during puncturing, are transmitted in response to the first forwarding request. The process may repeat, the redundancy bits being transmitted incrementally, in accordance with forwarding requests, as long as the incorrect block cannot be corrected.
The conventional ARQ and HARQ request for forwarding protocols are not directly applicable to cooperative networks as they do not take into account the presence of relays.
Recently, other request for forwarding protocols have been proposed for cooperative networks. e.g., the international application WO2008/024158 describes a relay-assisted HARQ protocol. According to this protocol, if the destination terminal does not succeed in decoding the data received from the source terminal in a first transmission interval, the source terminal will forward the data in a second transmission interval. In parallel, if the relay terminal has been able to decode the data received in the first transmission interval, it will forward the data to the destination terminal during the second transmission interval. By default, the relay terminal will remain silent during this second interval. Thus, it should be understood that the relay, and thereby diversity combining and/or code combining contributed thereby is/are taken into account only if the first transmission has failed.
A second request for forwarding protocol for cooperative networks was proposed in the article by B. Zhao et al., entitled “A Generalization of Hybrid-ARQ”, published in IEEE Journal on Selected Areas in Comm., vol. 23, no. 1, January 2005, pages 7-18. According to this protocol, at the beginning of the transmission interval the set D(s) of the nodes of the network having error-free knowledge of the data packet is taken into consideration. Thus, D(1) is reduced to the source terminal and D(2) comprises, in addition to the source terminal, the relay(s) having succeeded in decoding the data received in the first transmission interval. In the second transmission interval (s=2), from the relays belonging to D(2), the one closest to the destination terminal is selected, or else the one with the link having the highest instantaneous signal/noise ratio. The selected node ensures forwarding in the second transmission interval.
However, the proposed request for forwarding protocols have strong limitations.
Indeed, the protocol described in WO2008/024158 systematically makes use of data forwarding via the relay terminal, while the link between the same and the destination terminal may be deteriorated on the transmission resource used (frequency chunk in an OFDMA system). This transmission is then quite often useless.
The protocol described in the article cited selects the most suitable node for forwarding to the destination terminal, depending on a distance or signal/noise ratio criterion. However, if the link between the selected relay terminal and the destination terminal is deteriorated at the transmission resource used, this forwarding will probably be useless again.
The object of the present invention is to propose a forwarding protocol in a cooperative network which considerably reduces the probability of failure of the forwarding, in other words the probability that decoding of the data received by the recipient will lead to incorrect values. Similarly, the objective of the invention is to reduce the number of forwarding operations required for obtaining error-free decoding at the destination terminal.